Local Tetragonal Symmetry Research Articles

Order and disorder in cerium-rich ceria-zirconia solid solutions revealed from reverse Monte Carlo analysis of neutron and x-ray total scattering

A reverse Monte Carlo analysis of neutron and x-ray total scattering data from two ceria-zirconia samples of composition Ce0.75Zr0.25O2 is performed to analyze the distribution of cations and to examine the possibility of oxide-ion disorder. For the first material, heated in air under moderate conditions (800 °C), the structure is a single-phase solid-solution with the statistical distribution of cations, but a local tetragonal symmetry is found, consistent with the different coordination preferences of Ce and Zr. For the second material, heated under H2 at 1050 °C followed by reoxidation at 400 °C, the structure shows a considerable disorder, with evidence for oxygen interstitials (Frenkel-ion defects) and a non-statistical distribution of cations with significantly higher concentrations of like–like cation nearest neighbors, highlighting the existence of cation-rich nano-domains. The results highlight the dynamic nature of this solid-solution, with structural evolution upon thermal treatment, which is of relevance to understanding its stability under redox catalytic conditions in practical applications.

A highly-efficient approach for reducing phase transition temperature of VO2 polycrystalline thin films through Ru4+-doping

A low semiconductor-metal transition (SMT) temperature (TSMT) is highly desirable for advancing practical applications of VO2. The doping with tetravalent metal elements attracts little attention as an effective approach for reducing the TSMT of VO2. Here we investigate the effect of Ru-doping on the microstructure and SMT characteristics of VO2 polycrystalline thin films. X-ray photoelectron spectroscopy reveals that Ru ions exist in the tetravalent oxidation state in Ru-doped VO2 thin films. X-ray diffraction patterns and Raman spectra indicate that substitutional Ru4+ ions decrease the crystallite size of films and induce the local tetragonal symmetry in the framework of monoclinic VO2 lattice. As a result, Ru4+-doping reduces the TSMT of VO2 polycrystalline thin films at the rate of 10.5 °C/at%. Moreover, the room temperature resistivity of thin films decreases from 9.98 Ω·cm for undoped VO2 to 0.63 Ω·cm for Ru-doped one with Ru atomic concentration of 2.32 at%, which is ascribed to the narrowed band gap and enhanced density of states of the V 3d band.

Inversion in Mg1-xNixAl2O4 Spinel: New Insight into Local Structure.

A wide variety of compositions adopt the isometric spinel structure (AB2O4), in which the atomic-scale ordering is conventionally described according to only three structural degrees of freedom. One, the inversion parameter, is traditionally defined as the degree of cation exchange between the A- and B-sites. This exchange, a measure of intrinsic disorder, is fundamental to understanding the variation in the physical properties of different spinel compositions. Based on neutron total scattering experiments, we have determined that the local structure of Mg1-xNixAl2O4 spinel cannot be understood as simply being due to cation disorder. Rather, cation inversion creates a local tetragonal symmetry that extends over sub-nanometer domains. Consequently, the simple spinel structure is more complicated than previously thought, as more than three parameters are needed to fully describe the structure. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for many geophysics and energy-related applications, including prediction of deep seismic activity and immobilization of nuclear waste in oxides.

Role of polar nanoregions with weak random fields in Pb-based perovskite ferroelectrics

In relaxor ferroelectrics, the role of randomly orientated polar nanoregions (PNRs) with weak random fields (RFs) is one of the most puzzling issues of materials science. The relaxation time of polarization fluctuations of PNRs, which manifests themselves as a central peak (CP) in inelastic light scattering, is the important physical quantity to understand the dynamics of PNRs. Here, the angular and temperature dependences of depolarized and polarized CPs in 0.44Pb(Mg1/3Nb2/3)O3-0.56PbTiO3 single crystals with weak RFs have been studied by Raman and Brillouin scattering, respectively. The CPs observed in Raman scattering show the very clear angular dependence which is consistent with the local tetragonal symmetry. It is different from the well-known local rhombohedral symmetry with strong RFs for Pb(Mg1/3Nb2/3)O3. In Brillouin scattering, depolarized and polarized CPs show two relaxation processes corresponding to transverse and longitudinal fluctuations of PNRs. The remarkable slowing down towards the Curie temperature was observed for transverse fluctuations in local tetragonal symmetry.

Formation of new Tm3+ tetragonal symmetry optical centers in CaF2 hot-formed laser ceramics

By means of time-resolved site-selective spectroscopy the formation of new Tm3+ optical centers with modified local environment and presumably tetragonal local symmetry in CaF2 hot-formed laser quality ceramics is observed. The spectroscopic properties of these new Tm3+ optical centers are investigated and shown to differ strongly from that for regular tetragonal optical centers.

Monitoring the Structural and Vibrational Properties in RE-Doped SrTiO3 Ceramic Powders

X-ray diffraction and Raman spectroscopy were employed to analyze the effect of the rare earth (RE) doping (ionic radii and concentration) on the crystalline structure of Sr0.97RE0.03TiO3 (RE = Sm3+, Eu3+, Gd3+, Dy3+, Tm3+, Yb3+, Lu3+) and Sr1–xEuxTiO3 (x varying from 0.01 to 0.19) ceramic powders. We notice drastic changes in both structural and vibrational properties of the investigated samples for increasing dopant concentration into the SrTiO3 (ST) lattice. The results showed evidence of RE incorporation into both A- and B-sites of the ST structure. Also, tetragonal local symmetry (I4/mcm space group) was observed in RE-doped samples at room temperature from the cubic (Pm3m) phase exhibited by phase-pure ST ceramics. The results obtained for the Sr1–xEuxTiO3 ceramic powders showed that these samples undergo a clear cubic-to-tetragonal phase transition for x ≥ 0.13 at room temperature.

Crystal field analysis of Dy and Tm implanted silicon for photonic and quantum technologies.

We report the lattice site and symmetry of optically active Dy3+ and Tm3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence.

Anisotropic phonon coupling in the relaxor ferroelectric(Na1/2Bi1/2)TiO3near its high-temperature phase transition

The lead free relaxor ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}$ (NBT) undergoes a structural cubic-to-tetragonal transition near 800 K which is caused by the cooperative rotations of ${\mathrm{O}}_{6}$ octahedra. These rotations are also accompanied by the displacements of the cations and the formation of the polar nanodomains (PNDs) that are responsible for the characteristic dielectric dispersion of relaxor ferroelectrics. Because of their intrinsic properties, spontaneous polarization, and lack of inversion symmetry, these PNDs are also piezoelectric and can mediate an interaction between polarization and strain or couple the optic and acoustic phonons. Because PNDs introduce a local tetragonal symmetry, the phonon coupling they mediate is found to be anisotropic. In this paper we present inelastic neutron scattering results on coupled transverse acoustic (TA) and transverse optic (TO) phonons in the [110] and [001] directions and across the cubic-tetragonal phase transition at ${T}_{C}\ensuremath{\sim}800$ K. The phonon spectra are analyzed using a mode coupling model. In the [110] direction, as in other relaxors and some ferroelectric perovskites, a precipitous drop of the TO phonon into the TA branch or ``waterfall'' is observed at a certain ${q}_{\mathrm{wf}}\ensuremath{\sim}0.14$ r.l.u. In the [001] direction, the highly overdamped line shape can be fitted with closely positioned bare mode energies which are largely overlapping along the dispersion curves. Two competing lattice coupling mechanism are proposed to explain these observations.

Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors

In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature superconductors (HTCs) usually emerges in the presence of other fluctuating orders with similar or higher energy scales, thus instigating debates over their relevance for the SC pairing mechanism. In iron-based superconductors (IBSCs), local orbital fluctuations have been proposed to be directly responsible for the structural phase transition and closely related to the observed giant magnetic anisotropy and electronic nematicity. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy (ARPES) observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the SC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse the doping and temperature dependences of this band-splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity.

W LIII-edge XANES and EXAFS studies of Pb(Fe2/3W1/3)O3-PbTiO3 multiferroic ceramics

The local structure of Pb(Fe2/3W1/3)O3 (PFW) and Pb(Fe2/3W1/3)O3-PbTiO3 (PFW100xPT, x=0.10 and x=0.20) multiferroic ceramics was probed by X-ray absorption spectroscopy at the W LIII-edge. The analysis of XANES spectra did not show significative modifications in the intensity or energy of transitions as a function of temperature or PbTiO3 content. The fitting of EXAFS spectra of PFW samples revealed a local rhombohedral symmetry of W atoms around 120 K, which is consistent with the electrical properties of this compound. Concerning the local structure of W atoms in PFW100xPT samples, in all cases, at 290 K, a local tetragonal symmetry was determined although by X-ray diffraction a cubic local symmetry was expected.

Local structure around Fe ions on multiferroic Pb(Fe1/2Nb1/2)O3 ceramics probed by x-ray absorption spectroscopy

Local structure around Fe ions on Pb(Fe1/2Nb1/2)O3 ceramics was probed by x-ray absorption spectroscopy in order to settle the controversies about its structure. It is observed that the shell structure around Fe atoms exhibits a monoclinic local symmetry at 130 and 230 K, tetragonal local symmetry at room temperature, and cubic local symmetry at 410 K. Independently of the coordination, temperature, or symmetry, Fe-O mean bond-length does not vary significantly.

Electron-trapping centers and interstitials in chlorinatedSrCl2:Fesingle crystals

Electron-trapped ${\mathrm{Fe}}^{+}$-type centers, produced by x-ray irradiation at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and further annealing at higher temperatures in iron-doped ${\mathrm{SrCl}}_{2}$ single crystals grown in chlorine gas, have been investigated by electron paramagnetic resonance. The ${\mathrm{Fe}}^{+}(\mathrm{III})$ and ${\mathrm{Fe}}^{+}(\mathrm{IIIa})$ centers, produced by annealing at temperatures higher than $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, exhibit monoclinic local symmetry with the two $\stackrel{⃡}{g}$- tensor principal axes situated in the (110) plane slightly tilted away from the [001] and [1--10] directions, respectively. The ${\mathrm{Fe}}^{+}(\mathrm{IV})$ center, observed after several cycles of irradiation and annealing to $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, exhibits tetragonal local symmetry around $⟨100⟩$ and a well-resolved four-component structure, attributed to the superhyperfine interaction with a neighboring monovalent impurity ion. The presence and properties of the low symmetry radiation-induced ${\mathrm{Fe}}^{+}$ paramagnetic centers are attributed to trapping and the thermally activated movement of chlorine interstitials. Both precursor ${\mathrm{Fe}}^{2+}$ and resulting ${\mathrm{Fe}}^{+}$ centers are perturbed by these interstitials, which are introduced in ${\mathrm{SrCl}}_{2}:\mathrm{Fe}$ crystals during growth under a chlorine atmosphere.

Jahn-Teller effects in the mixed vanadate/phosphate crystalsTbV1−xPxO4(0<~x<~0.32)

X-ray-diffraction measurements were performed on mixed vanadate/phosphate single crystals of the type: ${\mathrm{TbV}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{\mathrm{O}}_{4}$ (where $x=0,$ 0.10, 0.18, 0.24, and 0.32) in order to investigate the effects of P doping on the Jahn-Teller phase transition. The transition temperature, below which the tetragonal symmetry of the lattice is lowered to orthorhombic, decreases with increasing P concentration. In order to explain the order parameters of the transition and the striking reduction in the transition temperature with increasing P content, a modified mean-field model was developed for the mixed system. A simple calculation assuming only a distribution in the value of the quadrupolar crystal-field parameter ${B}_{2}^{0}$ is unsatisfactory. Since the local tetragonal symmetry is broken due to the random distribution of phosphorous, a term involving the operator ${P}_{\mathrm{xy}}=\frac{1}{2}{(J}_{x}{J}_{y}{+J}_{y}{J}_{x})$ was introduced in the Hamiltonian that includes both the crystal-field and magnetoelastic effects. This term, in conjunction with the distribution in the magnitude of the corresponding crystal-field parameter, was essential in order to reproduce the observed temperature dependence of the order parameter (orthorhombic strain). The results of the diffuse scattering measurements performed on the $x=0.32$ crystal confirmed the existence of local strains that should give rise to variation in ${B}_{2}^{0}.$ The diffuse intensity due to strains having the symmetry associated with ${P}_{\mathrm{xy}}$ could not be resolved, however, because of the overlapping thermal diffuse scattering.

Single-ion anisotropy of localized-electron compounds

We present a computational analysis of the influence of taking the full multiplet structure into account in calculations of the single-ion contribution to the magnetocrystalline anisotropy (MCA) of $3d$ transition-metal (TM) ions in localized-electron compounds at $T=0$ K. For atoms with ${d}^{n}$ $(n=1--4,6--9)$ configurations, at sites with a local cubic, tetragonal, or trigonal symmetry the single ion MCA on the basis of the Hund's rule ground state term only [crystal field (CF) theory] is compared with the single-ion MCA on the basis of a fully relativistic first principles atomic theory including the intra-atomic $d\ensuremath{-}d$ interaction. Solid state effects are taken into account by effective crystal fields and the exchange field. Under certain, realistic, conditions the use of the full multiplet theory is shown to have a significant effect on the calculated single-ion MCA. We also discuss the effect on the overall cubic anisotropy constants for cubic crystals containing transition metal atom sublattices for which the $d\ensuremath{-}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}$ atoms are located on sites with a local tetragonal and trigonal symmetry. Possible refinements of the theory are discussed.

Quasimagnetic ordering of planar spins in a random anisotropy system: Dy(As0.35V0.65)O4

Distortions due to random As/V substitutions in Dy(${\mathrm{As}}_{0.35}$${\mathrm{V}}_{0.65}$)${\mathrm{O}}_{4}$ break local tetragonal symmetry and induce a uniaxial moment parallel to one or the other basal plane axis. This system thus provides an ideal random magnetic anisotropy antiferromagnet with equivalent competing axes and no dilution of the magnetic ions. Quasielastic neutron-scattering experiments of antiferromagnetic ordering at (100) showed a broad peak below 8 K with modest temperature dependence and a narrow peak that grew rapidly below 1.6 K. The narrow peak implies long range (800 \AA{}) spin correlations but is never resolution limited, ruling out a conventional magnetic transition. These results are qualitatively consistent with predictions of an unusual transition at the lower critical dimension for the random anisotropy model.

Origin of anomalous increase of hyperfine field in Dy(Ni xCu 1− x) 2Si 2

161Dy Mössbauer experiments at 3 K were performed on pseudo-ternary Dy(Ni x Cu 1− x ) 2Si 2 compounds of 0 ≤ x ≤ 0.15 in order to clarify an origin of anomalous increase of hyperfine field in the mixed compounds. The Mössbauer spectra of the mixed compounds are interpreted satisfactorily by three Zeeman subspectra. The concentration dependence of intensity ratio of three subspectra indicates that the hyperfine field depends strongly on the number of substituted Ni atoms in the nearest and next nearest transition metal sites which break the local tetragonal symmetry. The Dy atoms surrounded by the nearest neighbors containing both Cu and Ni atoms have large hyperfine fields close to the value of a pure |J z = − 15 2 〉 state. The local disturbance of crystalline electric field also brings forth the change of single-ion magnetic anisotropy. And then saturation magnetization decreases as the Ni content increases, since a part of Dy moments are arranged noncollinearly.

Van Hove exciton-cageons and high-Tc superconductivity

The low-temperature orthorhombic (LTO) phase transition in La$_{2-x}$Sr$_x$CuO$_4$ can be interpreted as a dynamic Jahn-Teller effect, in which the degenerate electronic states are associated with the large densities of states at the two van Hove singularities. The equations describing this phase are strongly nonlinear. This paper illustrates some consequences of the nonlinearity, by presenting a rich variety of exact nonlinear wave solutions for the model. Of particular interest are soliton lattice solutions: arrays of domain walls separating regions of local low-temperature tetragonal (LTT) symmetry. These arrays have a {\it macroscopic} average symmetry higher than LTT. These lattices can display either orthorhombic (`orthons') or tetragonal (`tetrons') symmetry, and can serve as models for a microscopic description of the dynamic JT LTO and high-temperature tetragonal phases, respectively.

Van Hove exciton-cageons and high-Tc superconductivity

Abstract The low-temperature orthorhombic (LTO) phase transition in La 2− x Sr x CuO 4 has been interpreted as a dynamic Jahn-Teller (JT) effect, with an octahedral tilt splitting the degeneracy of the two van Hove singularities. An explicit model of the LTO phase has been constructed as a soliton lattice: an array of domain walls separating regions of local low-temperature tetragonal (LTT) symmetry, but with a macroscopic average orthorhombic symmetry (‘orthons’). In this paper, the band structure is calculated for the simplest commensurate orthon superlattices, and it is shown that the degeneracy of the two van Hove singularities is broken in this phase. A similar splitting of the peak in the density of states (dos) is found for a CPA calculation of an arbitrary dynamic JT state. This splitting is essential to an understanding of the competition between structural instability and superconductivity in these materials. A third (unsplit) dos peak is also found, suggesting that the present model also provides a basis for explaining nanoscale phase disorder in these materials.

Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry

We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.

Superposition of glassy type and ferroelectric ordering in Rb1-x(ND4)xD2PO4

Abstract The influence of substitutional disorder on the phase transition behaviour in mixed crystals of Rb1−x(ND4)xD2PO4 is investigated by means of NMR-NQR line shape analysis on the ferroelectric side of the phase diagram for the compositions x = 0.10, 0.15, 0.20, and 0.22. It is shown that the average local tetragonal symmetry of the paraelectric phase is broken already far above Tc by the local random field which couples linearly to the local polarization. The onset of ferroelectric long range order is monitored via the average asymmetry parameter of the electrical field gradient tensor at the 87Rb sites.